Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007 Geocoding & GPS.
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Transcript of Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007 Geocoding & GPS.
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Summary
• Introduction to Geocoding• Geocoding: Concepts and Definitions• Relationship to other Census Processes• Approaches to Data Collection• NSO Benefits & Concluding Remarks
• Introduction to GPS• How GPS Works• Sources of Error & Accuracy• Selecting a GPS• Advantages & Disadvantages
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Introduction
• Many NSOs have a specialized coding scheme and understand geocoding as a dynamic process
• Clarification within the statistical community
• Expansion and discussion on components and methods within the process of geocoding
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Geocoding: Concepts and Definitions
• Definitions:• Conceptual
• Operational
• Geocoding vs Georeferencing
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
• Geocoding can be broadly defined as the assignment of a
code to a geographic location. Usually however, Geocoding
refers to a more specific assignment of geographic
coordinates (latitude,Longitude) to an individual address.
The purpose of this section is to introduce geocoding
concepts relevant for census mapping and the different
approaches to related data collection.
• Reference: UN Report of the Expert Group Meeting on Contemporary Practices in Census Mapping and Use of Geographical Information Systems (2007)
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Definition of Geocoding• Conceptual - 2 situations:
• The more general process of assigning geographic codes to features in a digital database.
• A GIS function that determines a point location based on an address. It could generally be expected that such point locations will be relatively precise (eg +/-2m) in accuracy and will be based upon use of GPS technology.
• Operational• Geocoding is the computer oriented process which
converts information about a unit from which statistical information is collected into a set of coordinates describing the geographic position of that unit
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Definition of Geocoding (cont.)• Operational Elements
• Collecting precise data at the level of point locations (or very low geographic level such as a city block) and assigning codes for use in dissemination.
• Coding the centroid, building corners, or building point of entry coordinates for a unit such as a block of land, building or dwelling
• Coordinates must contain latitude and longitude or standardized x and y points for gridded interpolation. A Z or Zed coordinate may represent altitude or elevation
• Codes cover each geographic unit and have a combinational relationship to distinguish different units (Enumeration Areas/Blocks)
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Geocoding vs. Georeferencing
• Geocoding• A GIS operation for converting street addresses into
spatial data that can be displayed as features on a map
• Georeferencing• Aligning geographic data to a known coordinate
system so it can be analyzed, viewed, and queried with other geographic data
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Relationship to Other Census Processes
• N.S.O Senior Managers must evaluate when the use of geocoding should be considered and consideration must be given due to:- range of hardware and software- availability of resources (street network reference layers, address database)- staff skill set - funding
- user demands
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Relationship to Other Census Processes (cont.)
• Movement into a fully GIS based approach to census mapping
• Generation of high quality maps for use in the collection phase
• Reduction of work required for updating maps for future censuses
• Aggregation of records into customized units for satisfying users’ requirements
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Census Enumeration & the Geocoding System
• Delineation irrespective of the existence of address• Ability to apply a geocode to any geographic areal unit
• Flexible Coding Scheme• Ability to incorporate future administrative divisions
• Pre-enumeration geocoding critical• links between GIS boundaries and tabular census data
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Census Hierarchies
• Internal political Boundaries
• Areal unit aggregation
• Resolution suitable to NSO needs and user demands
• Considers available datasets for continuous development
• The smaller area defined by the geocode the more flexible the results for subsequent users
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
BlocksEnumeration Areas
Country
District
Province
Locality
Given Country
Building Dwelling
Census Hierarchies
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Geocoding Classifications
• Disaggregation into Spatial Entities or Civil Divisions and Compatibility
1st Region Province
2nd District Municipality
3rd Town/Village
4th Dwelling
• Resultant geocoded units placed within a set of Latitude and Longitudinal boundaries
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Geocoding Classifications (cont.)
• Initial creation of Civil Divisions through digitizing or segmentation/pixel based-approaches
• Low to Zero levels of sampling through the accurate placing of coded units, but flexible enough to include changes
• Appropriate detail that fits with the boundaries of a geographic area for a given country
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Coding Scheme
Digits 1-2 = State code
250131402013
Digits 6-11 = Census Tract Code
Digits 3-5 = County Code
Digit 12 = Blockgroup code
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Data Collection Methods
• Two main methods:
• Direct Collection Approach
• Matching Approach
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Direct Collection Approach
• Digitizing from available topographic maps • Direct collection using field techniques
(ex.GPS)
Areas, Street, Dwelling
Global Positioning System (GPS)Digitizing from a topographic map
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Matching approach
• Using an Address locator database and street network database in a GIS
• Joining an address database to an existing spatial database for the area of interest
First A
venue
Second A
venue
Right of Street
Left of Street
#1 #99
#2 #100
address number
Main Street
First A
venue
Right of Street
Left of Street
#1 #99
#2 #100
#51
#32
Nodes
Second A
venue
Street SegmentStreet Network
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Data Maintenance
• Cleaning AddressesCleaning Addresses• Retaining only the key address elementsRetaining only the key address elements
• Establish a Matchcode (indicator of which Establish a Matchcode (indicator of which address elements will determine the address elements will determine the geocode)geocode)
• Eliminating extraneous charactersEliminating extraneous characters• Standardizing Spelling Standardizing Spelling
Record Street Address City State ZIPcode Latitude Longitude Areakey MatchCode1 344 East 63rd New York NY 10023 40.47 73.58 3502508100 AS0
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Staff Expertise Recommendations
Task/condition Direct collection Matching
Existence of digital base map for country Highly desirable Highly desirable
Statistical staff with expertise in use of GPS Essential Not Essential
Acquisition of large numbers of GPS receivers Essential Not Essential
Geo-referenced list of addresses or equivalent Not Essential Essential
Excellent address matching algorithms Not Essential Essential
Existence of a rational, consistent, and locally-recognized addressing system for housing units
Highly desirable Essential
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Geocoding: Benefits for NSO’s
• Improved map creation for the field
• Customizable map outputs for specified regional activities
• Coding techniques are transparent and transferable
• Fixates the groundwork for future statistical activities and coding schemes
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Concluding Remarks
• Technologies are accessible and allow delineation irrespective of the existence of address
• There is a need for an agreement on a definition of geocoding for census purposes
• Many available methods and technologies exist to support accurate geocoding frameworks
• Geocoding system is value-added for GIS based Spatial Analysis of Statistical Data
Prepared by UNSD. For Further Inquiries please contact [email protected]
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Global Positioning Systems (GPS)• Technology has revolutionized field mapping in recent years
• Prices of GPS receivers have dropped
• GPS methods have been integrated in many applications
• User groups are widespread (utilities management, surveying and navigation)
• GPS has contributed and advanced to improve field research in areas such as biology, forestry,geology, epidemiology and population studies
• GPS has become a major tool in census cartographic applications
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Global Positioning Systems (cont.)
• Preparation and updating of enumerator (EA) maps for census activities
• Location of point features such as service facilities or village centers
• Coordinates can be downloaded or entered manually into a digital mapping system or GIS, and can be combined with existing, georeferenced information
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
How GPS Works
• GPS receivers collect the signals transmitted from more than 24 satellites—21 active satellites and three spares. The system is called NAVSTAR, and is maintained by the U.S. Department of Defense
• The satellites are circling the earth in six orbital planes at an altitude of approximately 20,000 km. At any given time five to eight GPS satellites are within the “field of view” of a user on the earth’s surface
• The position on the earth’s surface is determined by measuring the distance from several satellites
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
The global positioning system (GPS)
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
The global positioning system (cont.)
• GPS satellites circle the Earth twice a day …
• The satellite signal:• Three kinds of coded information essential for
determining a position;
• The receiver:• 1. Calculates the distance to the first satellite user is
able to catch. • 2. Calculates the distance to a second satellite for
which it is able to catch a signal.• 3. Repeats the operation mentioned under point 2 with
a third satellite.
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
m e a s u r e dd is t a n c e
x
u
x
c
b
aHow GPS determines a location’s coordinates
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Sources of GPS signal errors
• Good visibility and bad visibility of satellites due to obstacles
• signal multipath• Uncontrollable sources of error over which the user does
not have control• Atmosphere delays • Receiver clock errors • Orbital errors
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
GPS Accuracy
• Inexpensive GPS receivers• Within 15 to 100 meters for civilian
applications. • Differential GPS reduces error further• Accuracy of about 3-10m can be achieved with
quite affordable hardware and shorter observation times.
• More expensive systems and longer data collection for each coordinate reading can yield sub-meter accuracy.
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Problems with GPS
• In dense urban settings, the possible error of standard GPS (standard ~15m up to 100 meters) may not be sufficient
• Differential GPS can be used for cross-checking GPS readings with other data sources• published maps• aerial photographs• sketch maps produced during fieldwork
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
DGPS ground station
DGPS mobile receiving station
GPS satellites
Correction signal
Space segment
Control segmentUser segment
Differential GPS
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Selecting a GPS Unit
• Commercially available GPS receivers vary in price and capabilities
• Technical specifications determine the accuracy by which positions can be achieved
• The more powerful a receiver, the more expensive it will be• In many mapping applications, the accuracy of standard
systems is quite sufficient• Receivers also vary in terms of user-friendliness, tracking
capabilities which are useful in navigation
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Summary: Advantages and Disadvantages of GPS
Advantages• Fairly inexpensive, easy-to-use field data collection• Modern units require very little training for proper use• Collected data can be read directly into GIS databases
minimizing intermediate data entry or data conversion steps
• Worldwide availability• Sufficient accuracy for many census mapping
applications—high accuracy achievable with differential correction
Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007
Summary: Advantages and Disadvantages of GPS
Disadvantages• Signal may be obstructed in dense urban or wooded areas• Standard GPS accuracy may require differential techniques • Differential GPS is more expensive, requires more time in
field data collection and more complex post-processing to obtain more accurate information
• A very large number of GPS units may be required for only a short period of data collection.